Although accurate at the time of publication, it is no longer being updated. The page may contain broken links or outdated information, and parts may not function in current web browsers.

Summary of "All Things Electric and Magnetic"

by David P. Stern

This is an excerpt of a summary of educational web sites summary of educational web sites, of those items describing "All Things Electric and Magnetic" (a simple listing of the items in the summary is here

=====================

(85) All Things Electric and Magnetic (Overview, September 2010)

This educational web-course is a first introduction, for students and interested web users, to the development and history of "classical" electricity and magnetism, covering key concepts and applications. Mathematics is limited to simple algebra and use of sine and cosine, and history serves as guiding thread.

Meant for the level of 8-10 grade, it (mostly) stops with E&M as it stood late in the 19th century-- as if electricity were a continuous fluid and electromagnetic
waves a continuous disturbance.

Suitable for independent study and home schooling, with special stress on electricity as used in the home. Linked to a wider site "From Stargazers to Starships" and to two related sites on magnetism of Earth and Space.

An elementary course on electricity and magnetism, focusing on electricity as it was viewed n the 19th century, as a "fluid". Quantum concepts such as electrons and photons are mentioned but not covered, and no calculus is used. The flow of electricity in wiring is compared to flow of water in plumbing, making voltage (a more familiar word than "potential") analogous to pressure and electric charge to volume (or to mass) of water.

(E4) Wiring Puzzles and the "Enigma" The "Enigma" encryption machine used by German armed services in World War II was essentially an elaborate switching link-up, through disks which scrambled the encoding of the alphabet multiple times. The signal ran through multiple disks, each with a circle of 26 electric contacts around each face and with scrambled connections inside. An odometer gear-link ensured that contacts changed after each letter in a preset order. The "Enigma" code was broken by Polish mathematicians, and later (as it evolved) by a British team and by an assembly of decrypting machines in the USA--a challenging task, not always successful, since the setting of the code was changed each day.

Keywords:
"Enigma", cryptology,"Bomba", Allan Turing.

(89) Electric power and safety

(E7) Electrical Safety (E8) Electric Power (E9) Fuses and Electric Shock Electric current can carry appreciable amounts of energy, to run machines, lights, computers etc. The rate of energy delivery ("power") is proportional to the voltage V supplied and to the current intensity I. If the current takes an unplanned path, this "short circuit" can cause electric shock or release enough heat to start a fire. Electric wiring is insulated to protect users, is "grounded" (e.g. in homes) and contains fuses and circuit breakers, including "ground fault interrupt" types.

(E10) Alternating Current (AC) (E11) The Reason for Using Alternating Current If constant flow of water in a pipe is an analogy to DC current, alternating current is like a flow periodically sloshing forward and back, delivering energy but no net water flow. The current and voltage both vary like a sine or cosine wave, with frequency (in US homes) of 60 cycles per second (50 in Europe), with peak amplitude I0 or V0. A resistor across a 110 volt AC line delivers the same energy as it would on a 110 V DC line, but the peak of the cycle V0 is 110 volts times the square root of 2 (=1.414..), 110 V being the "root mean square" (RMS) value

(E12) Electricity and Chemistry
(E13) Where Electricity and Chemistry Meet Since chemistry is due to electric forces between atoms, this self-contained course includes some general chemistry. Matter on Earth consists of atoms, often combined into molecules (examples listed), either by sharing electrons (covalent bond) or by one part of a molecules getting bound to an electron from he other (ionic bond). The charged fragments ("ions") are then strongly attracted by their opposite electric charges. Water greatly weakens such attraction, helping ionic compounds dissolve in water and (occasionally) separating them into charged ions. Ions are the foundation of "voltaic" cells and batteries which generate electricity, and a reverse process exists, electrolysis, in which an electric current breaks up an ionic solution chemically. The corrosion of metals in a humid atmosphere (or in seawater) is also related.

(E14) Early History of Electricity and Magnetism Static electricity and natural "lodestone" magnets were known in ancient Greece, while China discovered the magnetic compass. William Gilbert (1600) studied magnetic and electric forces, and proposed the Earth was a giant magnet. Stephen Gray distinguished conductors and insulators.

Keywords:
Amber, lodestone, William Gilbert, insulators, conductors.

(93) Electric Capacitance

(E15) Static Electricity(E16) Capacitance(E17) Capacitance and Stored Electrical Energy(E18) Pumping up the Voltage of a Static Charge Charles DuFay (1733) distinguished two types of electricity, and Ben Franklin assigned them + and - signs. He also recognized the electric nature of lightning and introduced lightning rods. Charles Coulomb with his "torsion balance" showed both magnetic and electric forces decreased with distance r from the magnetic pole or electric charge like 1/r2, and Dutch researchers in Leyden devised the "Leyden Jar" to hold more electric charge, an early capacitor. A capacitor and a rectifier (such as a solid state diode) can convert AC into DC, and capacitors can transmit voltage signals across a gap between different DC voltages (unwanted "pick-up" signals also use that route). Telegraph signals in cables under the ocean were degraded, in part due to capacitance.
Separating charged objects increases their energy, by overcoming their attraction, leading to hgher voltage. Static cling, Volta's "electrophorus" and Van de Graaff's high voltage generator all arose from this principle.

(E19) Electro-Magnetism(E20) Faraday and his "Lines of Force" Until 1820 the only magnetism known originated in permanent magnets. That year, by accident, Oersted in Denmark discovered magnetism produced by electric currents and Ampére deduced that perhaps the fundamental magnetic phenomenon was attraction (repulsion) by two parallel (anti-parallel) electric currents. Michael Faraday, son of a blacksmith who studied science while apprenticed to a bookbinder, visualized magnetism by "magnetic lines of force" (now termed "magnetic field lines") and used the new source of magnetism to cause a hanging wire dipping in a cup of mercury (which completed the circuit of a battery) to move around the end of a bar magnet.

(95) Early thoughts on the link between electricity and light with the text of Faraday′s lecture April 1846

(E21) Waves in Space"Thoughts on Ray Vibrations" 1846 lecture by Faraday
The velocity c of light was first estimated in 1676 by Ole Roemer, explaining why eclipses of the inner big moon of Jupiter were unevenly spaced. It also arose from the ratio of two observed units of electricity and magnetism. Light thus could be an electromagnetic wave--but a transverse ("sideways") wave, because of polarization. Faraday in an unscheduled talk in 1846 suggested that it could be a sideways oscillation of magnetic field lines; the text of the talk is also reproduced here. It was a wrong idea, but led to the correct one.

(E22) Electromagnetic Fields Faraday knew a current in a coil of insulated wire created magnetic effects similar to a bar magnet (later Joseph Henry actually built an electromagnet): did a reverse effect exist--could a bar magnet put inside a wire coil create a current? In 1831 he found it did, but only while the magnet was inserted or removed. Such "electromagnetic induction" led to alternating current technology, but also suggested that "empty" space around a magnet had a special property, which later was named "magnetic field".

(E23) Galvanometers A galvanometer measures electric currents by their magnetic fields or magnetic force. The old "tangent galvanometer" compares the force on a compass needle in the center of a coil carrying the measured current, to the force of the Earth's field. The widely used D'Arsonval galvanometer measures the magnetic force on a small coil between the poles of a magnet, by observing how far it can push a twisted spring, like a clock's mainspring. Somewhat similar forces rotate the central rotor of an electric motor.

(E24) Induction and Magnetic Flux
Electromagnetic induction produces a different kind of voltage--Faraday named it "electro-motive force" or EMF for short. EMF acts like a voltage source distributed around the electric circuit--no battery, and the voltage drop across any section obeys Ohm's law. The way the EMF is derived benefits from a close analogy between magnetic field lines and the flow of an "incompressible" fluid (water comes close). With water, the amount flowing into any closed volume (with no "sources" or "sinks") equals the amount flowing out. Magnetic flux has a similar property, if flowlines are replaced by magnetic field lines.
With this definition, the basic law of induction states that the EMF around any closed circuit is proportional to the rate at which the magnetic flux through it (flux from outside sources) changes.
A magneto is an electric generator using the motion of a permanent magnet near a coil, or vice versa. Common electric generators use an electromagnet, but magnetos came first; lawnmower engines and some bicycle lamps still use them.

(E25) Electric Power Technology The first magneto employing a rotating magnet was built in France in 1832, and would have created AC but for the action of a reversing switch ("commutator") on the axis of the rotating magnet. The development of electric generators (and motors) was delayed by over 40 years, until electric lights and motors were developed. The Edison company promoted DC, but George Westinghouse's "General Electric" prevailed with AC, which allowed transformers to raise and lower voltage.
Iron intensifies magnetization, up to a "saturation" level, Steel can be permanently magnetized, but any magnetic material exhibits "memory" after being magnetized ("hysteresis"). Hysteresis causes energy loss in transformers, whose iron core must reverse magnetization 120 times each second, and asymmetric saturation is the phenomenon behind fluxgate magnetometers, used widely in the lab and aboard spacecraft.

(E26) AC Impedance Resistors in a circuit absorb energy. Capacitors and inductors in an AC circuit also temporarily store energy over the course of each oscillation, and by this delay or advance the AC waveform. This leads to a generalized form of resistance known as AC impedance, depending on the frequency f and often expressed by using ω = 2πf (in units known as radians). An inductor of L "henry" units has impedance Lω, a capacitor of C "farad" has impedance 1/Cω, and combinations of both elements can exhibit resonances and need additional mathematical tools. A crystal radio uses a simple resonant circuit to tune to a station, after which a solid-state diode extracts the signal.

The Scotsman James Clerk Maxwell (1831-79) formalized and combined the laws of electricity and magnetism in space as a set of 4 "Maxwell's equations" (presence of material modifies them somewhat). By including among the currents producing of magnetic fields a "displacement current" associated with varying electric field, he showed (between 1861-73; the mathematics are not covered here) that electromagnetic waves could exist and propagate in space, with the speed and properties of light. Heinrich Hertz in 1883 actually generated such waves electrically, producing the first radio transmissions.

Soon after the discovery of electromagnetism ingenious inventors adapted it to transmit messages by electric signals along metal wires. The most lasting "telegraph" was the version devised by Samuel B.F.Morse, an American painter, whose "Morse Code" is still used. Telegraphs soon connected the world, even through insulated undersea cables. The patent for a telephone, reproducing sound by a membrane vibrated by a magnet, was given to Alexander Graham Bell, but others also contributed to the technology. Thomas Edison and Emil Berliner introduced the phonograph soon afterwards, and wireless telegraphy was developed by Giugliemo Marconi and used to send distress signals from the sinking "Titanic" n 1912.

A non-mathematical overview of research on the magnetic space environment around Earth, about 80 files, illustrated, includes Spanish version (MIntro.html) and one in French (incomplete), glossary, timeline, Q&A, teacher guidance, a history overview and articles "Birth of a Radiation Belt" and "Secrets of the Polar Aurora." Stresses history, also conceptual understanding and some basics such as electrons, ions and their motion in space, plasmas and energy.

A non-mathematical historical overview of the Earth's magnetism, written for the 400th anniversary of William Gilbert's book "De Magnete," covered in some detail. Also discusses electromagnetism, solar magnetism, dynamo theory, ocean floor magnetization, and magnetospheres of Earth and planets. Includes a long review "A Millennium of Geomagnetism," a detailed article on teaching magnetism in high schools and translations to Spanish, German and French.